Computer extension systems that allow use of a keyboard, monitor, and other computer peripherals an extended distance from a computer are becoming increasingly popular, with a number of extension systems in use today. In these systems, and in a typical case, the computer is located in a more hospitable or convenient environment than the keyboard, monitor, mouse, printer, or other peripherals, these peripherals being coupled to the computer by a single cable having a plurality of insulated conductors. Signal conditioning circuitry is coupled to one or both ends of the cable and conditions the digital and video signals attenuated by the extended length of the conductors conveying the discrete signals.
Problems with these systems generally involve induced noise, particularly video noise, and attenuation of the signals by the long cable lengths and RC roll-off filters typically used by manufacturers to reduce radiated noise as per FCC Part 15 requirements. Additionally, in single ended systems, offsets in ground potentials may, in some situations, be high enough to prevent some digital devices from transitioning to a "low" logic level.
In the signal conditioning network that receives the signals from the computer, particularly video and vertical and horizontal sync circuits, problems typically arise when video amplifier circuits are provided with ground and power potentials from the cable. In this instance, after traversing the length of the cable, the power potential is not at its original level due to line losses, and in operation may fluctuate due to switching demands, causing nonlinear responses from video amplifiers supplied by this power. Further, both power and ground potentials suffer induced noise from adjacent conductors. Here, regulation of power sources in this case is not feasible because of the lack of stable reference potentials and also because of the extra power and voltage drain a regulator would impose. Also, since the analog video signals are of a wide bandwidth, from 10 kHz to 150 mHz, the combined impedance from line inductance, capacitance, and resistance attenuates some frequencies more than others, causing nonlinear response from the video amplifiers and distortion of the video signals that degrade images displayed on the monitor.
As color VGA computers and monitors enjoy increased usage, color backgrounds have become more prevalent. These color backgrounds in an extension system require higher quality digital signals than dark or black backgrounds. Signal anomalies such as ringing, reflections, and attenuation of the high frequency components, or rise and fall times, of the video signals in the long conductors of the extension cable are much more apparent with a color background than a black background. Here, ringing and reflections typically cause "ghosts" of images on the monitor, while signal attenuation causes a leading edge, with respect to the horizontal sweep, of vertical lines of images to appear fuzzy and indistinct.
Accordingly, a need has arisen for an extension system capable of transmitting video signals with data signals applied to discrete conductors of a cable, with ringing, reflections, attenuation, induced noise, and other extraneous signals being suppressed or otherwise compensated.
The applicant has devised several extended range communications systems for coupling computer video and sync signals from a computer to a remotely located monitor. Notably, U.S. Pat. No. 4,885,718, issued on Dec. 5, 1989, to Asprey et al., discloses an extension system for extending computer video signals up to 150 feet to a monitor from the computer. Here, only monochrome signals are applied to the cable, with these signals driven at TTL levels. No attempt is made to bias the video levels to a selected bias, reduce amplitude of the signals to a significantly lower level, which causes less crosstalk than a stronger signal or to limit variance in current flow to the signal conditioning network at the monitor, which causes non-linear responses of the video amplifiers therein.
In patent application Ser. No. 07/447,010, filed on Dec. 5, 1989, by Asprey et al., now U.S. Pat. No. 5,193,200, color video signals are transmitted by TTL buffers on an extended cable up to 300 feet to a monitor. Here, the signals are attenuated by a roll-off filter prior to being applied to the cable and conditioned by Schmitt-triggered amplifiers in the receiving signal conditioning network. No attempt is made to bias the signal to a higher potential, significantly reduce the amplitude of the signal, or limit variance of current flow to the signal conditioning network at the monitor.
In patent application Ser. No. 07/488,710, filed on Mar. 5, 1990, by Asprey et al., an extension system is disclosed wherein analog video signals and horizontal and vertical sync signals are used to modulate current flow through an extended conductor of a cable. In this system, the video signals are terminated at close to the characteristic impedance of the conductor. Additionally, the video signals are impedance matched to the conductor of the cable and applied thereto at close to their original levels. No attempt is made to bias the video signals to a selected level, significantly reduce their amplitude, or limit variance of the current flow to the signal conditioning network at the monitor.
Another patent application, Ser. No. 07/555,580, filed on Jul. 19, 1990, by Asprey et al., discloses extended communications links up to 1,000 feet. Here, the video signals are digital signals which are biased to a selected level and reduced in amplitude. However, the digital video signals are provided with a voltage spike on leading edges thereof as precompensation that would degrade performance of the instant invention and negate the effect of reducing crosstalk provided by significantly reducing the amplitude of the video signal. Additionally, since this system is digital in nature, it cannot be used with the analog video signals of the instant invention since analog video signals are not compatible with the digital switching thresholds of the referenced application, which would result in analog video signals on the on the wrong side of these thresholds being lost.
With respect to the invention of the parent case of this application, which is disclosed herein, it has been found that temperature stabilization of the semiconductors of certain of the analog video circuitry is necessary where temperature variations between about 0.degree. F. and about 110.degree. F. or greater are encountered. This is brought about by virtue of the necessity for D.C. coupling of each of the transistors employed in the video amplifier circuit supplying the video monitor, the D.C. component of each transistor varying from approximately 750 mv when cold to about 450 mv when hot. With six transistor stages and an output transistor, the video amplifier works well when relatively cool, but when hot, the operating range of each transistor is shifted downward, resulting in increased current flow through each transistor when biased to a conductive state, and, as a result, an undesired D.C. voltage as an output to a monitor.
Thus, from the foregoing paragraph, it is to be appreciated that the amplifiers at the end of a video link which power the video inputs to a monitor present the problems indicated. There are, of course, three of these amplifiers, one for the red, blue, and green signals; and as suggested above, they require D.C. coupling which can produce detrimental effects by virtue of D.C. level drift which varies with temperature. It is the object of this invention to overcome the recited difficulties experienced with D.C. coupled amplifier driving a computer monitor.